The effectiveness of targeted anti-cancer therapeutics is frequently reduced by the acquisition of drug- resistance. A comprehensive understanding of the mutations compatible with oncogene function should define the mutations available to direct drug-resistance and provide a guide for the rational development of improved inhibitors with reduced probability of resistance. Traditional approaches to analyze the functional effects of mutations rely on randomly generated mutants and typically can only identify a handful of mutations with a strongly selected phenotype. We propose an approach to systematically analyze the functional effects of all possible single-nucleotide substitutions for entire oncogenes both in the presence and absence of inhibitors. Our approach will systematically define the positive or negative impact of each mutation on cell growth. Only mutations that are compatible with oncogene function should be available to drug resistance evolution. Therefore, identifying the set of functional mutations provides a powerful structural guide that can be incorporated in the early stages of future drug design efforts.